Process for coating of substrates with heat curable coating

ABSTRACT

A heat activatable adhesive or sealant organic resin composition comprising 
     (1) an epoxy resin containing at least two ##STR1## (2) a thermal initiator selected from the group consisting of diaryliodonium salts and diaryliodonium salts combined with a pinacol, and 
     (3) a thermoplastic adhesive material selected from the group consisting of polyesters, polyvinyl acetals, polyamides, butadiene-acrylonitrile copolymers, styrene-butadiene copolymers, styrene-isoprene copolymers, styrene-ethylene-butylene copolymers, ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers, ethylene propylene diene monomer and mixtures thereof. 
     The composition after application to the parts to be bonded or sealed forms a thermoset bond or seal on application of heat thereto, preferably by electromagnetic techniques including dielectric and induction heating.

This is a division of application Ser. No. 487,682, filed Apr. 22, 1983,which is a continuation-in-part of Ser. No. 317,672, Nov. 2, 1981, bothnow abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to thermoplastic adhesive and sealing organicresin compositions which, on application of heat, preferably in anaccelerated manner, crosslink to give a thermoset bond or seal.

The invention also relates to a process for forming a crosslinked bondor seal.

2. Description of the Prior Art

The concept of thermosetting or crosslinking resin adhesives is known inthe art. Many resin adhesives which undergo an irreversible chemical andphysical change and become substantially insoluble are well known.Thermosetting adhesives comprising both condensation polymers andaddition polymers are also known and examples include theurea-formaldehyde, phenol-formaldehyde and melamine-formaldehydeadhesives; epoxy, unsaturated polyester and polyurethane adhesives. Moreparticularly, U.S. Pat. No. 3,723,568 teaches the use of polyepoxidesand optional epoxy polymerization catalysts. U.S. Pat. No. 4,122,073teaches thermosetting resin obtained from polyisocyanates,polyanhydrides and polyepoxides. Crosslinking in these patents isachieved by reaction with available sites in the base polymers. U.S.Pat. No. 4,137,364 teaches crosslinking of an ethylene/vinylacetate/vinyl alcohol terpolymer using isophthaloyl biscaprolactam orvinyl triethoxy silane whereby crosslinking is achieved before heatactivation with additional crosslinking induced by heat afterapplication of the adhesive. U.S. Pat. No. 4,116,937 teaches a furthermethod of thermal crosslinking by the use of polyamino bis-maleimideclass of flexible polyimides, which compounds can be hot melt extrudedup to 150° C. and undergo crosslinking at elevated temperaturesthereabove. In these latter two patents, thermocrosslinking is alsoachieved by reactions of the particular crosslinking agent withavailable sites of the base polymers.

In substantially all of these thermosetting adhesives bond formation isdependent on the chemical crosslinking reaction which in most cases isaccelerated by means of heat to obtain the bond within a reasonableperiod of time. Further, in many cases, e.g., epoxy adhesives, two ormore components must be admixed just prior to the preparation of thebond. This necessitates a fast application since the crosslinkingreaction begins immediately upon admixture and is irreversible. Thus,there has been a desire for a one part thermosetting adhesive which canbe applied and thereafter triggered to cure on command.

Methods of achieving delayed tack are known in the art. See U.S. Pat.Nos. 2,653,880, 2,653,881 and 4,059,715 which teach the employment ofthermoplastic polymers containing slowly crystallizing segments.

On the other hand, thermoplastic adhesives, which are used in the formof solutions, dispersions or solids, usually bond by purely physicalmeans. Probably the most important means of applying thermoplasticadhesives is the hot melt method wherein bond formation occurs when thepolymer melt solidifies in position between adherends. The bondsobtained by this method reach their final strength faster than thoseobtained from solution type adhesives. Obviously, the thermal stabilityof the thermoplastic resin determines its potential usefulness as a hotmelt adhesive. In order for the thermoplastic to be used as a hot melt,it must also have a low melt viscosity, thus permitting application ofthe adhesive to the adherends at acceptable rates. Usually this meansthe polymer must have a low molecular weight. However, manythermoplastic materials cannot be employed as hot melts because they donot have sufficient cohesive strength at the low molecular weightsrequired for application to a substrate. For example, the low molecularweight polyolefins, especially low molecular weight, low densitypolyethylene, are widely used in hot melt adhesives for sealingcorrugated cartons, multi-wall bag seaming and the like, but they do nothave sufficient strength to be used in structural applications such asplywood manufacture. Further, they do not have sufficient heatresistance to be used for bonding components which are intermittentlyexposed to elevated temperatures such as under the hood automotiveapplications. That is, thermoplastic adhesives cannot be employed wherethe adhesive in situ is reexposed to elevated temperatures which willcause the adhesive to sag thereby allowing the bond to break.

In the prior art there are many two-part materials which are cured insitu at elevated temperature, e.g., epoxy and urethane resins. Thecuring times, however, are relatively long, thereby precluding on-lineproduction in a continuous operation. The curing time can besubstantially reduced by heating, but such methods are rarely used dueto the fact that external heating also causes substrate or adherends tobe heated. In the case of heat sensitive substrates and adherends, e.g.,thermoplastics, it can cause damage or distortion thereof.

OBJECTS OF THE INVENTION

One object of the instant invention is to produce a one part adhesivecomposition which is solventless. Another object of the invention is toproduce an adhesive composition which can be applied as a hot melt.Still another object of the instant invention is to produce an adhesivecomposition which is heat curable in a minimum time period. Yet anotherobject of the invention is to produce a process whereby an adhesivecomposition can be applied as a hot melt and thereafter cured by athermally triggered initiator to a thermoset adhesive at a more elevatedtemperature. Another objective is to provide a one part adhesivecomposition that can be applied as a free film, tape or as a preformedgasket. Other objects will become apparent from a reading hereinafter.

DESCRIPTION OF THE INVENTION

The present invention relates to a heat activatable adhesive compositioncomprising

(1) an epoxy resin containing at least two ##STR2## (2) a thermalinitiator selected from the group consisting of diaryliodonium salts anddiaryliodonium salts in combination with a pinacol, and

(3) a thermoplastic adhesive material selected from the group consistingof polyesters, polyvinyl acetals, polyamides, butadiene-acrylonitrilecopolymers, styrene-butadiene copolymers, styrene-isoprene copolymers,styrene-ethylene-butylene copolymers, ethylene-vinyl acetate copolymers,ethylene-ethyl acrylate copolymers, ethylene propylene diene monomer andmixtures thereof.

The composition after application to the parts to be bonded or sealedforms a thermoset bond or seal on application of heat thereto,preferably by electromagnetic techniques including dielectric andinduction heating.

The composition after application to the parts to be bonded or sealedforms a thermoset bond or seal on application of heat thereto,preferably by electromagnetic heating techniques including dielectricand induction heating. Although the composition and process taughtherein is operable to form a thermoset bond or seal, the invention forthe most part for reasons of brevity will be explained in terms ofadhesive bonding.

The epoxy resin to be used in the composition of the invention comprisesthose materials possessing at least two epoxy, i.e., ##STR3## groups.These compounds may be saturated or unsaturated, aliphatic,cycloaliphatic, aromatic or heterocyclic and may be substituted withsubstituents, such as chlorine, hydroxyl groups, ether radicals and thelike.

The term "epoxy resin" when used herein and in the appended claimscontemplates any of the conventional monomeric, dimeric, oligomeric orpolymeric epoxy materials containing a plurality, at least 2, epoxyfunctional groups. Preferably, they will be members of classes describedchemically as (a) an epoxidic ester having two epoxycycloalkyl groups;(b) an epoxy resin prepolymer consisting predominately of the monomericdiglycidyl ether of bisphenol-A; (c) a polyepoxidized phenol novolak orcresol novolak; (d) a polyglycidyl ether of a polyhydric alcohol; (e)diepoxide of a cycloalkyl or alkylcycloalkyl hydrocarbon or ether; or(f) a mixture of any of the foregoing. To save unnecessarily detaileddescription, reference is made to the Encyclopedia of Polymer Scienceand Technology, Vol. 6, 1967, Interscience Publishers, N.Y., pages209-271, incorporated herein by reference.

Suitable commercially available epoxidic esters are preferably,3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (Union CarbideERL 4221, Ciba Geigy CY-179); as well asbis(3,4-epoxy-6-methylcyclohexylmethyl)adipate (Union Carbide ERL 4289);and bis(3,4-epoxycyclohexylmethyl)adipate (Union Carbide ERL 4299).

Suitable commercially available diglycidyl ethers of bisphenol-A areCiba Geigy Araldite 6010, Dow Chemical DER 331, and Shell Chemical Epon828 and 826.

A polyepoxidized phenol formaldehyde novolak prepolymer is availablefrom Dow Chemical DEN 431 and 438, and a polyepoxidized cresolformaldehyde novolak prepolymer is available from Ciba-Geigy Araldite538.

A polyglycidyl ether of a polyhydric alcohol is available from CibaGeigy, based on butane-1,4-diol, Araldite RD-2; and from Shell ChemicalCorp., based on glycerine, Epon 812.

A suitable diepoxide of an alkylcycloalkyl hydrocarbon is vinylcyclohexene dioxide, Union Carbide ERL 4206; and a suitable diepoxide ofa cycloalkyl ether is bis(2,3-epoxycyclopentyl)-ether, Union Carbide ERL0400.

Other examples include the epoxidized esters of the polyethylenicallyunsaturated monocarboxylic acids, such as epoxidized linseed, soybean,perilla, oiticica, tung, walnut and dehydrated castor oil, methyllinoleate, butyl linoleate, ethyl 9,12-octadecadienoate, butyl9,12,15-octadecatrienoate, butyl eleostearate, monoglycerides of tungoil fatty acids, monoglycerides of soybean oil, sunflower, rapeseed,hempseed, sardine, cottonseed oil and the like.

The thermal initiators employed when the epoxy resin is an epoxycontaining at least two ##STR4## are thermal initiators selected fromdiaryliodonium salts, either per se or combined with a pinacol. Thediaryliodonium salts operable herein as thermal initiators incombination with a pinacol are those set out in U.S. Pat. No. 4,238,587,and it is understood that so much of the disclosure therein relative tothe diaryliodonium salts is incorporated herein by reference. That is,the diaryliodonium salts which can be utilized in the practice of theinvention are shown as follows:

    [(R).sub.a (R.sup.1).sub.b I].sup.+ [Y].sup.-,             (1)

where R is a C.sub.(6-13) aromatic hydrocarbon radical, R¹ is a divalentaromatic organic radical, and Y is an anion, a is equal to 0 or 2, b isequal to 0 or 1 and the sum of a+b is equal to 1 or 2. Preferably, Y isan MQ_(d) anion where M is a metal or metalloid, Q is a halogen radicaland d is an integer equal to 4-6.

Radicals included within R of formula (1) can be the same or differentaromatic carbocyclic radicals having from 6 to 20 carbon atoms, whichcan be substituted with from 1 to 4 monovalent radicals selected fromC.sub.(1-8) alkoxy, C.sub.(1-8) alkyl, nitro, chloro, etc. R is moreparticularly phenyl, chlorophenyl, nitrophenyl, methoxyphenyl, pyridyl,etc. Radicals included by R¹ of formula (1) are divalent radicals suchas ##STR5## where Z is selected from --O--, --S--, ##STR6## R² isC.sub.(1-8) alkyl or C.sub.(6-13) aryl, and n is an integer equal to 1-8inclusive.

Metals or metalloids included by M of formula (1) are transition metalssuch as Sb, Fe, Sn, Bi, Al, Ga, In, Ti, Zr, Sc, V, Cr, Mn, Cs, rareearth elements such as the lanthanides, for example, Cd, Pr, Nd, etc.,actinides, such as Th, Pa, U, Np, etc., and metalloids such as B, P, As,Sb, etc. Complex anions included by MQ_(d) are, for example, BF₄ ⁻, PF₆⁻, AsF₆ ⁻, SbF₆ ⁻, FeCl₄ ⁻, SnCl₆ ⁻, SbCl₆ ⁻, BiCl₅ ⁻⁻, etc.

Some of the diaryliodonium salts which can be used in the practice ofthe invention are as follows: ##STR7##

These thermal initiators are added to the system in an amount rangingfrom 1 to 10% by weight of the epoxy resin.

The substituted or unsubstituted pinacols operable herein as a thermalinitiator have the general formula: ##STR8## wherein R₁ and R₃ are thesame or different substituted or unsubstituted aromatic radicals, R₂ andR₄ are substituted or unsubstituted aliphatic or aromatic radicals and Xand Y which may be the same or different are hydroxyl, alkoxy oraryloxy.

Preferred pinacols are those wherein R₁, R₂, R₃ and R₄ are aromaticradicals, especially phenyl radical and X and Y are hydroxyl.

Examples of this class of compounds include, but are not limited to,benzopinacol, 4,4'-dichlorobenzopinacol, 4,4'-dibromobenzopinacol,4,4'-diiodobenzopinacol, 4,4',4",4"'-tetrachlorobenzopinacol,2,4-2',4'-tetrachlorobenzopinacol, 4,4'-dimethylbenzopinacol,3,3'-dimethylbenzopinacol, 2,2'-dimethylbenzopinacol,3,4-3',4'-tetramethylbenzopinacol, 4,4'-dimethoxybenzopinacol, 4,4',4",4"'-tetramethoxybenzopinacol, 4,4'-diphenylbenzopinacol,4,4'-dichloro-4",4"'-dimethylbenzopinacol,4,4'-dimethyl-4",4"'-diphenylbenzopinacol, xanthonpinacol,fluorenonepinacol, acetophenonepinacol,4,4'-dimethylacetophenone-pinacol, 4,4'-dichloroacetophenonepinacol,1,1,2-triphenyl-propane-1,2-diol, 1,2,3,4-tetraphenylbutane-2,3-diol,1,2-diphenylcyclobutane-1,2-diol, propiophenone-pinacol,4,4'-dimethylpropiophenone-pinacol,2,2'-ethyl-3,3'-dimethoxypropiophenone-pinacol,1,1,1,4,4,4-hexafluoro-2,3-diphenyl-butane-2,3-diol.

As further compounds according to the present invention, there may bementioned: benzopinacol-mono methylether, benzopinacol-mono-phenylether,benzopinacol and monoisopropyl ether, benzopinacol monoisobutyl ether,benzopinacol mono (diethoxy methyl) ether and the like.

The pinacol is added to the composition in amounts ranging from0.01-10%, preferably 0.1-5%, by weight based on the weight of the epoxyresin.

The diaryliodonium salts are operable per se to initiate thecrosslinking reaction but preferably are used in combination with apinacol disclosed herein due to the faster cure rate.

The thermal initiator can be added to the system in various ways. Thatis, the thermal initiator, per se, can be admixed with the epoxy groupmember. Furthermore, the thermal initiator can be dissolved or suspendedin well known commercially available solvents such as dibutyl phthalate;ketones, e.g., acetone and methylethyl ketone or chlorinatedhydrocarbons such as methylene chloride, and then added to the system.

The thermoplastic adhesive material component of the heat activatableadhesive organic resin composition can be made up of various saturatedand unsaturated thermoplastic polymers and copolymers, the term"copolymers" including terpolymers, tetrapolymers, etc.

These thermoplastic adhesive materials along with the remainder of theorganic resin composition can be applied in hot melt form as a one partadhesive. These thermoplastic adhesive materials are composed of 100%non-volatile materials, i.e., containing no water, solvent or othervolatile carriers. They are solid or liquid at room temperature butbecome more fluid at elevated temperatures, thereby allowing for easyapplication. The thermoplastic adhesive materials operable hereininclude, but are not limited to, polyamides, polyvinyl acetals andpolyester resins, ethylene-vinyl acetate (EVA) copolymers,ethylene-ethyl acrylate (EEA) copolymers, butadiene-acrylonitrilecopolymers and styrene-ethylene-butylene copolymers. Some of the newermaterials of the more conventional "rubber" variety are the blockcopolymers, styrene-butadiene or styrene-isoprene sold under thetradename "Kraton". These thermoplastic adhesive materials are sometimesused in conjunction with secondary components including waxes,plasticizers, reactive diluents, fillers and anti-oxidants. Waxfunctions as a non-volatile solvent for the thermoplastic adhesivematerial and reduces melt viscosity. Plasticizers, reactive diluents orliquid modifiers are used to a limited extent to formulate flexibility,specific wetting and viscosity characteristics into hot melt compounds.Fillers are used for cost reduction, color control and to improvecohesive properties. Antioxidants are used to retard oxidation duringcompounding and application.

One thermoplastic adhesive material useful in the adhesive compositionsof the present invention includes those thermoplastic segmentedcopolyesters disclosed in U.S. Pat. No. 4,059,715, incorporated hereinby reference. Another particularly suitable thermoplastic copolyestercomprises radicals of adipic acid, a C₃₆ dimer acid formed from linoleicacid (available under the trade designation "EMPOL 1010" from EmeryIndustries), 1,4-cyclohexane dimethanol and a poly(oxytetramethylene)glycol having a molecular weight of 2,000 and a hydroxyl equivalent ofapproximately 56 (available under the trade designation "Polymeg 2000"from E. I. DuPont Co.).

Other thermoplastic adhesive materials which are useful in the adhesivecompositions of the present invention include other thermoplasticpolyesters (e.g., that available under the trade designation "5096" fromCooper Polymers, Inc.), thermoplastic polyurethanes (e.g., thatavailable under the trade designation "Q-thane PH 56" from K. J. QuinnCo., Inc.), thermoplastic polyamides (e.g., that available under thetrade designation "Coramid 2430" from Cooper Polymers, Inc.),"Elvamides" available from DuPont and "Macromelt" available from Henkel;thermoplastic rubbers (e.g., those available under the trade designation"Kraton 1101" and "Kraton 1107" from Shell Chemical Co.) and ethylenevinylacetate (e.g., that available under the trade designation "Elvax40" from E. I. DuPont de Nemours Co., Inc. and "Ultrathene" availablefrom USI). Still other thermoplastic adhesive materials operable as acomponent in the adhesive organic resin composition include, but are notlimited to, butydiene-acrylonitrile copolymers available under the tradedesignation "Hycar" from B. F. Goodrich, urethane-acrylates,urethane-epoxides and urethane-polyenes. In addition, otherthermoplastic materials are polyvinyl acetals such as polyvinyl formaland polyvinyl butyrals. The thermoplastic adhesive material is presentin the composition in amounts ranging from 1-95% by weight of thecomposition with the balance being the epoxy resin.

In the instances where the thermoplastic adhesive material containsethylenic unsaturation, e.g., styrene-butadiene copolymers and ethylenepropylene diene monomer, it is possible for curing of both the epoxyresin and the ethylenically unsaturated thermoplastic to occur onheating. When the thermoplastic adhesive material is void of suchgroups, it merely acts as a matrix for the crosslinkable epoxy resin andprovide additional adhesive properties.

The compositions of the present invention may, if desired, include suchadditives as antioxidants, inhibitors, fillers, antistatic agents,flame-retardant agents, thickeners, thixotropic agents, surface-activeagents, viscosity modifiers, plasticizers, tackifiers and the likewithin the scope of this invention. Such additives are usuallypreblended with the epoxy resin prior to or during the compounding step.Operable fillers which can be added to the system to reduce cost includenatural and synthetic resins, glass fibers, wood flour, clay, silica,alumina, carbonates, oxides, hydroxides, silicates, glass flakes,borates, phosphates, diatomaceous earth, talc, kaolin, barium sulfate,calcium sulfate, calcium carbonate, wollastonite, carbon fibers and thelike. The aforesaid additives may be present in quantities up to 500parts or more per 100 parts of the organic resin composition by weightand preferably about 0.005 to about 300 parts on the same basis.

Additionally, scavengers and antioxidants such as hydroquinone,pyragallol, phosphorous acid, triphenyl phosphine, tert-butylhydroquinone, tert-butyl catechol, p-benzoquinone,2,5-diphenylbenzo-quinone, 2,6-di-tert-butyl-p-cresol, etc., are addedto the system in conventional amounts ranging from 0.001 to 2.0% byweight of the epoxy resin.

The heating step is usually carried out for a period of 1 second to 30minutes at a temperature of 70°-200° C., preferably 90°-170° C. which issufficient to fully cure the composition to a solid thermoset adhesiveor sealant product.

The heating step using a thermal initiator to cure the adhesive organicresin composition can be accomplished in several ways. In simplestystems, the adhesive composition can be applied by manual means to anadherend, contacted with another adherend and the assembled systemheated in an air oven until a thermoset bond results.

Additionally and preferably, electromagnetic heating can be utilized asa faster and more efficient means of curing, especially where thesubstrates to be bonded are plastic materials. In addition to theformation of high strength bonds, electromagnetic bonding techniques aidin (a) fast bond setting times, and (b) automated part handling andassembly.

In practicing the instant invention, electromagnetic heating can beemployed with the adhesive composition herein to adhere (1) plastic toplastic, (2) plastic to metal and (3) metal to metal. For example,dielectric heating can be used to bond (1) and (2) supra if the adhesivecomposition contains sufficient polar groups to heat the compositionrapidly and allow it to bond the adherends. Inductive heating can alsobe used to bond (1), (2) and (3). That is, when at least one of theadherends is an electrically conductive or ferromagnetic metal, the heatgenerated therein is conveyed by conductance to the adhesive compositionthereby initiating the cure to form a thermoset adhesive. In theinstance where both adherends are plastic, it is necessary to add anenergy absorbing material, i.e., an electrically conductive orferromagnetic material, preferably in fiber or particle form (10-400mesh) to the adhesive composition. The energy absorbing material isusually added in amounts ranging from 0.1 to 2 parts by weight, per 1part by weight of the adhesive organic resin composition. It is alsopossible to impregnate the plastic adherend at the bonding joint withparticles of the energy absorbing material in order to use inductiveheating, but care must be exercised that the plastic is not distorted.

The particulate electromagnetic energy absorbing material used in theadhesive composition when induction heating is employed can be one ofthe magnetizable metals including iron, cobalt and nickel ormagnetizable alloys or oxides of nickel and iron and nickel and chromiumand iron oxide. These metals and alloys have high Curie points(730°-2,040° F.).

Electrically conductive materials operable herein when inductive heatingis employed include, but are not limited to, the noble metals, copper,aluminum, nickel, zinc as well as carbon black, graphite and inorganicoxides.

There are two forms of high frequency heating operable herein, thechoice of which is determined by the material to be adhered. The majordistinction is whether or not the material is a conductor ornon-conductor of electrical current. If the material is a conductor,such as iron or steel, then the inductive method is used. If thematerial is an insulator, such as wood, paper, textiles, syntheticresins, rubber, etc., then dielectric heating can also be employed.

Most naturally occurring and synthetic polymers are non-conductors and,therefore, are suitable for dielectric heating. These polymers maycontain a variety of dipoles and ions which orient in an electric fieldand rotate to maintain their alignment with the field when the fieldoscillates. The polar groups may be incorporated into the polymerbackbone or can be pendant side groups, additives, extenders, pigments,etc. For example, as additives, lossy fillers such as carbon black at aone percent level can be used to increase the dielectric response of theadhesive. When the polarity of the electric field is reversed millionsof times per second, the resulting high frequency of the polar unitsgenerates heat within the material.

The uniqueness of dielectric heating is in its uniformity, rapidity,specificity and efficiency. Most plastic heating processes such asconductive, convective or infrared heating are surface-heating processeswhich need to establish a temperature within the plastic andsubsequently transfer the heat to the bulk of the plastic by conduction.Hence, heating of plastics by these methods is a relatively slow processwith a non-uniform temperature resulting in overheating of the surfaces.By contrast, dielectric heating generates the heat within the materialand is therefore uniform and rapid, eliminating the need for conductiveheat transfer. In the dielectric heating system herein the electricalfrequency of the electromagnetic field is in the range 1-3,000megahertz, said field being generated from a power source of 0.5-1,000kilowatts.

Induction heating is similar, but not identical, to dielectric heating.The following differences exist: (a) magnetic properties are substitutedfor dielectric properties; (b) a coil is employed to couple the loadrather than electrodes or plates; and (c) induction heaters couplemaximum current to the load. The generation of heat by inductionoperates through the rising and falling of a magnetic field around aconductor with each reversal of an alternating current source. Thepractical deployment of such field is generally accomplished by properplacement of a conductive coil. When another electrically conductivematerial is exposed to the field, induced current can be created. Theseinduced currents can be in the form of random or "eddy" currents whichresult in the generation of heat. Materials which are both magnetizableand conductive generate heat more readily than materials which are onlyconductive. The heat generated as a result of the magnetic component isthe result of hysteresis or work done in rotating magnetizable moleculesand as a result of eddy current flow. Polyolefins and other plastics areneither magnetic nor conductive in their natural states. Therefore, theydo not, in themselves, create heat as a result of induction.

The use of the eletromagnetic induction heating method for adhesivebonding of plastic structures has proved feasible by interposingselected eletromagnetic energy absorbing materials in an independentadhesive composition layer or gasket conforming to the surfaces to bebonded, electromagnetic energy passing through the adjacent plasticstructures (free of such energy absorbing materials) is readilyconcentrated and absorbed in the adhesive composition by such energyabsorbing materials thereby rapidly initiating cure of the adhesivecomposition to a thermoset adhesive.

Electromagnetic energy absorbing materials of various types have beenused in the electromagnetic induction heating technique for some time.For instance, inorganic oxides and powdered metals have beenincorporated in bond layers and subjected to electromagnetic radiation.In each instance, the type of energy source influences the selection ofenergy absorbing material. Where the energy absorbing material iscomprised of finely divided particles having ferromagnetic propertiesand such particles are effectively insulated from each other by particlecontaining nonconducting matrix material, the heating effect issubstantially confined to that resulting from the effects of hysteresis.Consequently, heating is limited to the "Curie" temperature of theferromagnetic material or the temperature at which the magneticproperties of such material cease to exist.

The electromagnetic adhesive composition of this invention may take theform of an extruded ribbon or tape, a molded gasket or cast sheet orfilm. In liquid form it may be applied by brush to surfaces to be bondedor may be sprayed on or used as a dip coating for such surfaces.

The foregoing adhesive composition, when properly utilized as describedhereinafter, results in a solvent free bonding system which permits thejoining of metal or plastic items without costly surface pretreatment.The electromagnetically induced bonding reaction occurs rapidly and isadaptable to automated fabrication techniques and equipment.

To accomplish the establishment of a concentrated and specificallylocated heat zone by induction heating in the context of bonding inaccordance with the invention, it has been found that theelectromagnetic adhesive compositions described above can be activatedand a bond created by an induction heating system operating with anelectrical frequency of the electromagnetic field of from about 5 toabout 30 megacycles and preferably from about 15 to 30 megacycles, saidfield being generated from a power source of from about 1 to about 30kilowatts, and preferably from about 2 to about 5 kilowatts. Theelectromagnetic field is applied to the articles to be bonded for aperiod of time of less than about 2 minutes.

As heretofore mentioned, the electromagnetic induction bonding systemand improved electromagnetic adhesive compositions of the presentinvention are applicable to the bonding of metals, thermoplastic andthermoset material, including fiber reinforced thermoset material.

The following examples are set out to explain, but expressly not limit,the instant invention. Unless otherwise noted, all parts and percentagesare by weight.

Strength properties of adhesive in shear by tension loading were run inaccord with ASTMD 1002-64 based on one inch square of lapped area.

EXAMPLE 1 Preparation of Diphenyliodonium Tetrafluoroborate

20 g of silver tetrafluoroborate were dissolved in 20 g of water in abeaker at 60° C. with stirring. 33.52 g of 97% diphenyliodonium chloridewere dissolved in 720 g of water in another beaker at 60° C. withstirring. The silver tetrafluoroborate solution was slowly poured intothe diphenyliodonium chloride solution and the AgCl precipitate wasremoved by filtration. The filtrate was refrigerated for 2 daysresulting in the formation of white crystals. The filtrate was thawedand refiltered. The resulting white crystal solids from this filtrationwere washed with water, air dried and then vacuum dried over night toobtain 11.1 g of white crystals. The filtrate was reduced to 2/3 itsvolume in a Bucchi rotovapor and then refrigerated. After thawing atroom temperature the filtrate was refiltered and the white crystals werecollected as set out above. The two resultant long white needle productsweighed 28.6 g and had a melting point in the range 132°-137° C.

Analysis. Calcd. for C₁₂ H₁₀ BF₄ I: C, 39.16; H, 2.72; B, 2.94; F,20.67; I, 34.51. Found: C, 39.15; H, 2.64; B, 3.04; F, 20.55; I, 34.98.

EXAMPLE 2 Curing of Heat Activatable Adhesive Composition

8 g of a polyamide having a softening point in the range 87°-100° C.sold under the tradename "Macromelt-6071" by Henkel Adhesives Co. weredissolved in a 50/50 methyl alcohol-methylene dichloride solvent at 40°C. 2 g of a bisphenol-A diepoxide sold under the tradename "DER-331" byDow Chemical Co. was added to the solvent along with 0.06 g benzopinacoland 0.06 g of diphenyliodonium tetrafluoroborate. After all the adhesivecomposition materials were dissolved in the solvent, the solvent wasremoved by vacuum at 30° C. 5 samples were prepared for lap shear testby applying the thus formed adhesive composition to an as received coldrolled steel adherend which was clamped to a similar adherend to form a1/2" lap. The clamped adherends were placed in a forced air ovenmaintained at 160° C. for 20 minutes. After removal from the oven andcooling to room temperature, the lap sheer was 2410±370 psi.

A control run without any bisphenol-A diepoxide resulted in a lap shearof 420±70 psi.

The following examples in TABLE I show the strength properties ofvarious heat activatable adhesive compositions in shear. In all theexamples, the thermoplastic adhesive material was dissolved in asolvent, i.e., 50/50 MeOH/CH₂ Cl₂, or CH₂ Cl₂ at 40° C. followed by thedissolution of the epoxy resin. Next, the thermal initiator is dissolvedin the solvent. The solvent was removed by vacuum and the remainingsolid heat-activatable adhesive composition was applied between twoadherends in a 1/2" lap. 5 sets of test samples were made up for eachadhesive composition. The adherends were clamped together by binderclamps and placed in a forced air oven maintained at 160° C. for 20minutes. The test samples were removed from the oven, unclamped andallowed to cool to room temperature before lap shear measurements (psi)were taken. The average of the five test values is shown in TABLE I.

                                      TABLE I                                     __________________________________________________________________________    Conventional Curing of Adhesive Compositions                                                       Epoxide Group Member (wt. %)                             Example                                                                            Adhesive Thermoplastic Material                                                                      1,4 Butanediol     Lap Shear, psi.sup.a           No.  Type       Wt. %                                                                              DER 331.sup.b                                                                        Diglycidyl Ether                                                                       Initiator (wt. %)                                                                       Steel.sup.c                    __________________________________________________________________________    1    Polyvinyl butyral.sup.d                                                                  100  --     --         --        720                          2    "          74.40                                                                              24.80  --       Onium salt.sup.e, 0.79                                                                  1,700                          3    "          73.89                                                                              24.63  --       Onium salt.sup.e, 0.74                                                                  2,380                                                               Benzopinacol 0.74                        4    Ethylene - 28% vinyl                                                                     100  --     --         --        220                               acetate copolymer.sup.f                                                  5    Ethylene - 28% vinyl                                                                     79.05                                                                              19.76  --       Onium salt.sup.e, 0.59                                                                  1,120                               acetate copolymer.sup.f         Benzopinacol 0.59                        6    Polyamide.sup.g                                                                          100  --     --         --        710                          7    "          79.05                                                                              --     19.76    Onium salt.sup.e, 0.59                                                                  1,540                                                               Benzopinacol 0.59                        __________________________________________________________________________     FOOTNOTES:                                                                    .sup.a Cured at 160° C. for 20 minutes, then lap shear measured at     R.T., all laps 1/2" except where noted;                                       .sup.b DER331 = BisphenolA diepoxide, commercially available from Dow         Chemical;                                                                     .sup.c All on as received steel, except where noted;                          .sup.d Butvar B76, average molecular weight 45,000 to 55,000, Monsanto;       .sup.e Diphenyliodonium tetrafluoroborate;                                    .sup.f Ultrathene 63604, softening pt 106° C., commercially            available from USI Chemicals;                                                 .sup.g Elvamide, melting point 145 to 160° C., DuPont.            

The compositions herein for the most part are solids at room temperaturewhich can be used as reactive hot melt adhesives. That is, the solidcomposition can be heated to a molten or plastic mass at a temperaturebelow the decomposition temperature of the thermal initiator and beplaced between 2 substrates to be adhered. Upon cooling to a lowertemperature the adhesive solidifies to a thermoplastic adhesive withproperties sufficient to adhere the substrates. Thereafter, theassembled works can be heated to a higher temperature to trigger thethermal initiator and form a thermoset adhesive between the substrates.A further refinement can be the use of the solid composition in the formof film, tape or gasket to be placed between adherends. This is shown inthe following examples.

EXAMPLE 8

100 g of polypropylene glycol (MW=1025 g/mole) were added dropwise overa 6-hour period to a flask containing 34 g of toluene diisocyanate in anitrogen atmosphere. The reaction was continued with stirring for 4 daysat room temperature. The resultant chain-extended isocyanate terminatedproduct will hereinafter be referred to as diisocyanate adduct A.

EXAMPLE 9

Into 100 g of methylene chloride solvent was charged 100 g of an epoxyresin containing 357 g/eq of OH, commercially available from ShellChemical Co. under the tradename "Epon-1001F", 0.1 g of dibutyl tindilaurate and 40.89 g of diisocyanate adduct A from Example 39. Thereaction was stirred at room temperature for 24 hours and monitored byIR until no isocyanate could be detected resulting in a thermoplastic,epoxy pendant, urethane containing compound. To 7.5 g of said compoundin methylene chloride was added 2.5 g of a liquid bisphenol-A epoxymethacrylate, commercially available from Shell Chemical Co. under thetradename "EPOCRYL Resin-12", 0.3 g of diphenyliodoniumtetrafluoroborate and 0.3 g of benzopinacol. After stirring until ahomogeneous mixture was obtained, the methylene chloride solvent wasremoved under vacuum leaving a heat activatable, solid, hot meltadhesive composition. The composition was applied in a 2-4 mil thicknessbetween 2 cold-rolled steel adherends. The adherends were clampedtogether and heated at 70° C. for 5 minutes and then cooled to roomtemperature. On removal of the clamps the thermoplastic compositionmaintained adherence between the assembled work. The assembled work wasreheated to 160° C. for 20 minutes resulting in a thermoset adhesivebetween the adherends.

EXAMPLE 10

10 g of a bisphenol-A epoxy methacrylate, commercially available fromShell Chemical Co. under the tradename "EPOCRYL-12", and 0.1 g ofbenzopinacol were dissolved in CH₂ Cl₂ at room temperature. The solventwas removed by vacuum at room temperature and the remaining solidheat-activatable adhesive composition was applied between two asreceived cold-rolled steel adherends in a 1/2" overlap. Five sets ofsamples were made up. The adherends were clamped together by bindingclamps and placed in a forced air oven maintained at 160° C. for 20minutes. The test samples were removed from the oven, unclapped andallowed to cool to room temperature. The material cured but had suchpoor adhesive properties it was too weak to test for lapshear.

EXAMPLE 11

1 g of "EPOCRYL-12" was admixed with 9 g of a polyamide having asoftening point in the range 132°-144° C. and commercially availablefrom Henkel Adhesive Co. under the tradename "Macromelt-6238" and 0.01 gof benzopinacol in a solvent, i.e., 50/50 MeOH/CH₂ Cl₂, at 30° C. Thesolvent was removed by vacuum and the remaining solid heat-activatableadhesive composition was applied between two as received cold-rolledsteel adherends in a 1/2" overlap. Five sets of test samples were madeup. The adherends were clamped together by binding clamps and placed ina forced air oven maintained at 160° C. for 20 minutes. The test sampleswere removed from the oven, unclamped and allowed to cool at roomtemperature before lapshear measurements (psi) were taken. The averageof the five test values for lapshear was 1,354 psi.

These 2 examples show the increased adhesion afforded by the inclusionof a thermoplastic adhesive material into the acrylate curable system.

The following examples show the ability to rapidly form a cured adhesiveusing RF dielectric or induction heating.

EXAMPLE 12

7.5 g of a polyvinylbutyral, commercially available from Monsanto underthe name "Butvar B-76" and 2.5 g of a bisphenol-A diepoxide, sold underthe tradename "DER-331" by Dow Chemical Co., were dissolved in amethylene chloride solvent along with 0.08 g of diphenyliodoniumtetrafluoroborate. After all the adhesive composition materials weredissolved in the solvent, the solvent was removed by vacuum at 35° C.Five samples were prepared for lapshear tests by applying the thusformed adhesive composition to fiberglass reinforced polyester adherendsin a 1/2" overlap. The adhesive was cured by radio frequency radiationat 200 volts and 1.0 amps of direct current for 2 minutes. An averagelapshear strength of 459 psi. was obtained.

EXAMPLE 13

8 g of a butadiene-14% of acrylonitrile copolymer, commerciallyavailable from B. F. Goodrich under the name "Hycar-1041",1,4-butanediol diepoxide, 0.06 g of diphenyliodonium tetrafluoroborateand 0.06 g of benzopinacol were dissolved in a methylene chloridesolvent to which was added 2.5 g of iron powder (minus 300 mesh). Thesolvent was removed by vacuum at room temperature. The remainingreactive adhesive composition was placed between fiberglass reinforcedpolyester adherends in a 1/2" lap and the curing was carried out on a 2kw EMABond generator model EA-20 at 90-100% load for 2 minutes. The lapjoint was placed in methylene chloride solvent to see if the adhesivedissolved (remained uncured). After 2 hours the adhesive did notdissolve showing that a cured adhesive bond was obtained.

We claim:
 1. A process for forming a coating on a substrate whichcomprises admixing a heat curable composition comprising(1) an epoxyresin containing at least two ##STR9## (2) a thermal initiator selectedfrom the group consisting of diaryliodonium salts and diaryliodoniumsalts in combination with a pinacol, and (3) a thermoplastic adhesivematerial selected from the group consisting of polyesters, polyvinylacetals, polyamides, butadiene-acrylonitrile copolymers,styrene-butadiene copolymers, styrene-isoprene copolymers,styrene-ethylene-butylene copolymers, ethylene-vinyl acetate copolymers,ethylene-ethyl acrylate copolymers, and mixtures thereof,coating saidadmixture on a substrate and heating said coating in the range 70°-200°C. to effect curing.
 2. The process according to claim 1 wherein theheating step is carried out by electromagnetic heating.
 3. The processaccording to claim 2 wherein the electromagnetic heating is bydielectric heating.
 4. The process according to claim 2 wherein theelectromagnetic heating is by induction heating.
 5. A sealant resultingfrom the process of claim
 1. 6. An adhesive resulting from the processof claim
 1. 7. A process for adhering two substrates which comprisescontacting said substrates with a solid, heat curable compositioncomprising(1) an epoxy resin containing at least two ##STR10## (2) athermal initiator selected from the group consisting of diaryliodoniumsalts and diaryliodonium salts in combination with a pinacol, and (3) athermoplastic adhesive material selected from the group consisting ofpolyesters, polyvinyl acetals, polyamides, butadiene-acrylonitrilecopolymers, styrene-butadiene copolymers, styrene-isoprene copolymers,styrene-ethylene-butylene copolymers, ethylene-vinyl acetate copolymers,ethylene-ethyl acrylate copolymers, and mixtures thereof,heating saidcomposition to a temperature whereas said composition becomessufficiently plastic to act as a thermoplastic adhesive between saidsubstrates, said temperature being below the decomposition temperatureof the thermal initiator and thereafter heating to a higher temperature,sufficient to decompose the thermal initiator and form a thermosetadhesive between the substrates.